How do I assign subnet IDs? Working through CCENT labs ch 17

[staticat]

I'm working through the CCENT labs and I'm confused how they are assigning subnets. I've pasted the instructions and Table 1 (what I have to complete) and Table 3 (the answers) below. 

I understand why the lowest subnet for network 10.0.0.0/24 is 10.0.0.0 (subnet 1), but why is the lowest and next available subnet for 10.0.0.0/25 (subnet 2) assigned 10.0.1.0 and not 10.1.0.0 or 10.0.0.1? Is this a matter of preference? Could I assign these 7 subnets like: 10.0.0.0, 10.1.0.0., 10.2.0.0., etc?

[Jon Marshall · Accepted solution]

You can't use 10.0.0.1 because that is part of 10.0.0.0/24 and 10.0.0.0/25 is also part of 10.0.0.0/24 so that is not useable either. 

The question is really saying what is the next available subnet after 10.0.0.0/24 and that has to be 10.0.1.0 because you have used up the whole of the 10.0.0.0/24 subnet already. 

You could use 10.1.0.0 etc. but then the subnets would not follow on from each other ie. between 10.1.0.0/24 and 10.0.0.0/24 there are a lot of available /24s to use. 

Jon

[staticat]

So is 10.0.1.0 considered a lower number, and that is why 10.1.0.0 is wrong?

[Jon Marshall]

To be honest Cisco questions are never really that clear to me. 

I took it to mean use the next available subnet after 10.0.0.0/24 and that is 10.0.1.0/24. 

Jon

[Deepak Kumar · Accepted solution]

Hi,
I am not sure that I can explain or not. But Try to understand the question:
There is many points are hidden in the above question:

1: What subnet is offered to you: 10.0.0.0/8

2. What is R1 LAN Subnet: 10.0.0.0/24 means from IP Address: 10.0.0.1 to 10.0.0.254

3. Now what is next available subnet from the given subnet: 10.0.1.0 

4. So R3 LAN Subnet will be: 10.0.1.0/25 means 10.0.1.1 to 10.0.1.126

Regards,

Deepak Kumar

[Richard Burts]

It seems to me that the key to this discussion is the instruction that in assigning subnets the lowest available subnet should be used. After you have used 10.0.0.0/24 then the lowest available subnet is 10.0.1.0.

And yes 10.0.1.0 is considered a lower number that 10.1.0.0.

HTH

Rick